Detectors to sense the breakage of glass are well known in the art. See, for example, U.S. Pat. No. 4,853,677. While the detector disclosed therein is responsive to high frequency audio sound, such as breakage of glass, as well as low frequency sound, such as the opening of doors and windows, the detector therein cannot discriminate between the opening of a window or the closing of a window. In an automobile, because the volume of air in the interior is trapped, an impact on the window (without breakage thereof) or on the panel lining the interior results in a low frequency audio sound or a pressure wave. Other actions which might produce a low frequency positive pressure wave would be the closing of a door, trunk or hood.
U.S. Pat. No. 4,928,085 discloses a detection device to detect pressure waves at low and infrasonic frequencies. The detected pressure signal is compared to a threshold and an alarm is generated in response thereto. In addition, a combination pressure and sound activated device is also disclosed (see FIG. 4 thereof). A sound discriminator can be used to reduce the incidents of false alarms and glass breakage detectors.
As previously discussed, the detection of intrusion into a vehicle is unique in that the detector must generate an alarm signal in response to the breakage of the glass or of only certain types of pressure changes.